General microbiology: capsule demonstration: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · General microbiology: capsule demonstration

Sample item

Alpha-haemolytic Gram-positive diplococci are isolated from sputum. When a drop of the specimen is mixed with type-specific antiserum and methylene blue, the capsule appears sharply outlined and swollen under the microscope. What is this reaction called?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Quellung (Neufeld) reaction

The Quellung reaction occurs when type-specific anticapsular antibody binds the polysaccharide capsule of Streptococcus pneumoniae, making it appear swollen and refractile. It is used to serotype pneumococci. The other named tests identify entirely different properties.

Option by option

  • A. Nagler reaction

    The Nagler reaction shows lecithinase of Clostridium perfringens and is inhibited by specific antitoxin.

  • B. CAMP test

    The CAMP test identifies group B streptococci by synergistic haemolysis with staphylococcal beta-lysin.

  • C. Quellung (Neufeld) reaction (correct)

    The apparent capsular swelling after antiserum binding is the Quellung reaction used to serotype pneumococci.

  • D. Elek gel precipitation test

    The Elek test demonstrates toxin production by Corynebacterium diphtheriae using antitoxin-impregnated paper.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, Quellung (Neufeld) reaction. The Quellung reaction occurs when type-specific anticapsular antibody binds the polysaccharide capsule of Streptococcus pneumoniae, making it appear swollen and refractile.

Why is "Nagler reaction" not the answer?

The Nagler reaction shows lecithinase of Clostridium perfringens and is inhibited by specific antitoxin.

Why is "CAMP test" not the answer?

The CAMP test identifies group B streptococci by synergistic haemolysis with staphylococcal beta-lysin.

Why is "Elek gel precipitation test" not the answer?

The Elek test demonstrates toxin production by Corynebacterium diphtheriae using antitoxin-impregnated paper.

What concept does this question test?

It tests General microbiology: capsule demonstration in Microbiology. It is used to serotype pneumococci. The other named tests identify entirely different properties.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General microbiology: capsule demonstration?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.